This invention addresses a special problem of excessive graininess in incremental printing of images initially prepared for printing in terms of four-color separations or equivalent. Four-color separations are in effect a device-space language, the language traditionally used for input data in make-ready procedures preliminary to printing-press operation.
(a) “Ink thinkers”—Although printing-press techniques are definitely not the field of the present invention, color printing-press technology has existed far longer and has long been one of the most popular areas of vocational training. Millions of people the world around have been trained in the traditional field of graphic arts, and of letterpress or offset printing make-ready.
That training typically includes a very extensive set of protocols for design of colorant in a piece to be printed. Some of those protocols relate to the somewhat more traditional prepress technology of the process camera and its follow-on procedures—i. e. making and stripping up four-color negatives (including the so-called “bumping” of exposures to control final hardcopy quality), and exposure of printing plates through the finished negatives to produce press masters.
Other protocols relate to more-modern procedures for generating negatives (or plates directly) in computer systems. All these procedures are highly elaborated, so that people who have learned them are actually able to remember a remarkable number of interactions among the pragmatic effects that result from various combinations of processing adjustments or refinements.
In particular, overprinting of black or dark gray ink with a tone of some chromatic ink—such as, often, magenta—is known in the traditional graphic arts, especially for printing-press make-ready, as producing a striking and even compelling visual effect. The experience generated in the mind or eye of the final viewer is different from, and deeper than, might be predicted from the modern calorimetric theory that has grown up around incremental printing.
Indeed, that modern theory by and large almost denies that there can be any physical meaning to a color produced by adding a chromatic colorant to a full or very nearly full black. Yet the artisans trained in the traditional printing-press arts know better, and may sometimes describe the result of such specialized overprinting tactics as imparting extra “kick” or “punch” to an image—or as a “nonspectral” effect.
Thus, generally speaking, these trained people have been taught to think in terms of ink—the ink that will go into the press from cans, and that will be summoned out of the press and onto the paper (or other printing medium) by the exact character of images formed on the plates. Even though it is in principle a rather roundabout mental gymnastic, these artisans have studied and learned to conceptualize the final outcome of a printing job not on the basis of any theoretical perceptual color theory but rather on the basis of ink. They are the “ink thinkers”.
The corps of personnel trained in these matters have learned quite sophisticated ways to produce fine effects in output hardcopies. It requires substantial investment in months and years of experience to be able to foretell how various small differences in film and plate preparation will impact—several process steps and often one or two departments later—upon a stream of final hardcopies from a press.
Many of the individuals thus trained are extremely competent, and are justifiably proud of their abilities. Rightfully, they are well regarded as craftswomen and craftsmen in the highest sense.
As the technology of incremental printing has matured and acquired a certain dominance for short-run work, naturally many of those people have been attracted to this new field. It is only natural that those people should expect to bring with them the extremely extensive approaches that they spent their formative years learning. Those approaches are indeed remarkably powerful in the context of printing-press operations.
(b) Loss of ink-thinking—Those approaches unfortunately cannot be realized within the conventional framework of the more modernly arrived incremental printing—which instead of inking concepts has focused upon theoretically better-grounded perceptual-color concepts. Despite the fact that such modern approaches may be better in theory, they are quite alien to those artisans who came up through the traditional printing arts.
Furthermore, the capability in which they are so experienced, and so well educated, is simply inaccessible in incremental printing. It is not merely that they must learn a new language or a new set of mental habits: these artisans are in general readily capable of such effort.
The problem is greater. The conventional computer programs and procedures developed to control incremental printers simply refuse to give over fully effective color control to personnel who wish to enter color specifications in the form of four-color separations, for a printing job. As noted above, many incremental-printing control systems essentially deny physical meaning to a great category of the color specifications developed in that way.
(There are other reasons to preserve direct control over quantity of black, as through four-color separations or other forms of device-space color specs. For instance, some printers do not support the use of true black on some media—as for example in the case of printers with pigmented black and dye-based color inks.)
What the printing-press corps of artisans has long been able to create with a printing press, simply cannot be done in a high-quality manner through incremental printing. It is true that some incremental-printing control systems can accept four-color separations as inputs; however, as subsection (e) below will explain, these systems in at least one way and sometimes two distinct ways defeat the expectations of the ink-thinkers.
That subsection describes the customary use of perceptual color spaces in incremental printing. In preparation for that discussion, however, some additional facts about undercolor removal, graininess and black replacement will be helpful.
(c) Undercolor or gray-component removal, and graininess—Incremental printers are somewhat more sensitive than printing presses to generation of excessive graininess in highlight and midtone regions. This problem arises in rendition of the neutral component of a color.
By “neutral component” is meant to encompass not only light grays to midrange grays but also a portion of any color that has some common amount of all three subtractive primaries (cyan C, magenta M and yellow Y). It is well known that this common amount of three color inks—in purest calorimetric principle—can be replaced by a like amount of a single ink, namely black.
Such a replacement has recognized benefits. It reduces the total quantity of ink used and the associated expense, and in theory also provides a greater guarantee of actual colorimetric neutrality for the nominally neutral component.
Consequently such replacement, known as “undercolor removal” (UCR) or “undercolor replacement” has been made an automatic feature of many color-management schemes commonly used in incremental printing. Unfortunately terminology in this field varies considerably, but UCR usually refers to removal and replacement of the entire common quantity of the three subtractive primaries.
A related but somewhat more-general phrase, “gray-component replacement” (GCR), is usually used to refer to removal and replacement of all or only part of the available common quantity of those three primaries. For purposes of simplicity in the remainder of this document, unless otherwise indicated by context all references to “UCR” will mean “UCR or GCR, or both, as appropriate”.
As mentioned above, automatic UCR has been built into many printers. It is also well known, however, that in incremental-printing practice such a replacement has important drawbacks:
For highlight and low-midtone regions, the resulting black ink dots—as compared with the calorimetrically equivalent grouping of color primary dots—intrinsically must be spaced relatively farther apart, thus appearing as graininess. Furthermore in some kinds of incremental printers (particularly inkjet printers) this is modernly aggravated by a trend for black pens to produce lower-quality image features than other pens.
(In the inkjet field, printheads are often called “pens” although they are far more complicated and sophisticated than the familiar unitary-writing-element model of e. g. a manually used ballpoint or fountain pen. For purposes of this document, except as otherwise indicated by context the term “pen” is to be understood as encompassing any incremental-printing printhead—whether inkjet or not, and even including a pagewide array.)
Reasons for this trend are not fully understood. It has been speculated that artifacts in black features are more visible simply because contrast is higher for black features generally, relative to the background.
In the printing-press world, by comparison, UCR is not a major problem. There, placement is extremely precise and accurate; and moreover the inks and the printing media used in the printing-press field are much more forgiving giving of representation of neutral components by black ink.
UCR-generated graininess, in particular, is at a very acceptable level in printing-press operations. Regrettably this cannot be said of UCR in the incremental-printing environment.
(d) “Black replacement” or “BR”—Therefore in the incremental-printing field it has also become commonplace to institute certain limitations or exceptions to the use of UCR. In particular, it is known to pause in the process of establishing color rendition and replace quantities of tentatively established black, in highlights and midtones, with equivalent amounts of chromatic color.
Such a replacement is seen in the previously mentioned patent of Perumal and Dillinger, who refer to “black replacement” (BR) and even to an extension of that approach, “black and secondary color replacement”. The phrase “tentatively established”, however, is very important to understanding here—as Perumal and Dillinger do not reverse the entire regimen of replacements.
That is, they do not go so far as to remove black from any input color specification, i. e. anything that might be called “original” black. Hence for purposes of this document their use of phrases such as “black replacement”, and also the short acronym “BR” as used in the present document, are strictly limited to the particular Perumal and Dillinger form of replacement, which means only replacement of an intermediate numerical value, representing a tentatively established quantum of black, in their calculations.
BR heretofore has been used only in those types of incremental printing that are based on original images generated or received through typical computer-graphics programs—that is, programs which operate either in additive-primary (red R, green G and blue B) color space or in a perceptual color space such as CIELAB. A few other incremental-printing systems are controlled by alternative color spaces which are related to perceptual spaces or are special-purpose perceptual/device-space hybrids, as for instance the “hue plus gray” space introduced in the previously mentioned patent of Dillinger.
Replacement of black by chromatics may possibly be known in the printing-press environment too, but if so it is discretionary and fully under control of the artisan. The number of personnel—if any—familiar with this technique for highlight regions is surely smaller than those familiar with, say, overprinting of a chromatic tone on top of black. The use of such replacement may be regarded as a subtle effect, for extra-smooth highlights, as compared with such overprinting—which is instead a dramatic effect.
Thus a good working knowledge of such replacement technique, if known at all in the printing-press environment, may perhaps be confined to those workers needing enhancements for special applications such as fine-art reproduction projects. Because UCR is not usually a major problem in the printing-press environment, replacement of black by chromatics is not a simple necessity of life there—as it is in incremental printing.
Therefore this discussion is not at all intended to suggest that black replacement should be eliminated, when performing incremental printing based on device-space color specifications: very much the contrary is the case. In incremental printing heretofore, however, BR has been invoked automatically by a perceptual-color stage—discussed below—and it is this grounding in customary perceptual theory that is objectionable.
(e) The colormap—This shorthand terminology “the colormap” is often used in referring to mapping or conversion of input color specifications into a perceptual or hybrid space for manipulation. For instance some computer-graphics programs receive inputs in the form of additive-RGB specifications and perform a transformation upon those numbers to derive equivalent perceptual values.
Then after complete manipulation the program reconverts the resulting colors into subtractive CMYK for printing. Programs of the colormap type dominate the incremental-printing field and in fact are sophisticated and extremely useful.
In fact such approaches have also been used in some computer programs that may be called “graphic arts” programs—which are specifically written to accept inputs in the form of subtractive device-space color data, and generate outputs that are nominally suitable for incremental printing. In other words, these programs are able to receive for instance four-color (usually CMYK) separations such as mentioned earlier.
Unfortunately passing such data into a colormap creates problems that are especially important and troublesome. These programs, just as in the RGB-input case, conventionally begin by converting the four-color separation data (or more generally subtractive device-space data) into a perceptual or hybrid space for manipulation. In fact sometimes these programs begin with an intermediate step of transforming CMYK inputs into RGB space, and then as usual converting to perceptual or hybrid space.
What is important to this discussion is not the exact order of events, but rather only the fundamental premise underlying the events. That premise is that perceptual space is the sole rational environment for color manipulations, preparatory to final conversion into (or back into) printing-device space for operation of some apparatus.
This premise represents a major misstep in the implementation of four-color separations by incremental printing. When four-color separation data are converted into any conventional perceptual space, of necessity the conversion process discards information that is often crucial to the image-design thinking which previously went into formulation of the four-color separations.
This discarding—of a critical part of the intelligence embedded in those device-space specifications—is one of the two ways, mentioned in subsection (b) above, in which customary incremental-printing systems defeat the intentions of the ink-thinker. More specifically, once a four-color separation set has been translated into perceptual terms, the originally intended allocation of inks to image regions can no longer be reconstructed.
Consequently every special colorant effect created by the graphic artist or printing make-ready technician is destroyed in this process. As that technician or artist would see it, the job has simply been ruined.
(f) Gamut, and nonideal inks—The conclusions in the preceding paragraph are true even for a color that in principle could have been reproduced within the perceptual system—i. e. colors within the theoretical gamut of the perceptual system. In other words, what a printing device will do with such a color, if the device is controlled on the basis of this sort of perceptual system, is to make an at least plausible approximation of the specified color.
The artist or technician has wholly lost control of the quantity of black actually printed. Still, in defense of the system in such cases it is fair to say that within the limits of conventional perceptual rendition theory—if not in terms of what can be perceived actually—the output color does correspond to the specified color.
Even in this case there are various degrees of failure of the correspondence. For example, a so-called “process black” obtained by adding cyan C, magenta M and yellow Y inks—or in another notation “CMY” composite black—may often appear very slightly brownish rather than dead black; whereas on the other hand at least one particular black ink actually displays a very slight magenta cast.
In such situations it is fair to say that neither inking is ideal; however, this is precisely where the skill of the ink-thinker becomes invaluable. Such a craftsperson considers it an essential part of the work to become familiar with the actual behavior of one or sometimes several given sets of inks, in a given printing system—an artistry largely lost upon users of perceptual systems.
Thereafter, for each printing job the ink-thinking artisan first judiciously selects the preferred visual effect and then specifies the corresponding ink—or combination of inks—within a selected ink set, to implement that desired effect. Of course such finesse is entirely foreclosed in prior-art systems that operate on a colormap basis.
As noted above, however, this is only the first of the two blows which a conventional incremental system deals to the experienced artisan who has come up through the printing industry. It is not the end of the story.
The second defeat comes from the way in which a conventional perceptual-space system, in incremental printing, defines color gamut and relates that gamut to device spaces. In such systems an image region that is endowed with a fully imprinted black has exhausted the available gamut of the system.
This comment should not be misunderstood as a statement about perceptual colorimetry in general. Of course when a spectrophotometer or proper colorimeter is used to measure the perceptual calorimetric content of an actually printed field of “full black” with an overprinted quantity of, say, magenta, such measuring apparatus clearly reports a calorimetric quantity which is very plainly different both from “full black” alone and from the overprinted quantity of magenta alone.
What is under discussion here is only the limited forward, Peprinting perceptual-color theoretical formulation that is customary in incremental printing. Within the confines of this conventional preprinting analysis, “black” is a unique color that occupies the bottom tip of a three-color-dimensional solid representing the system gamut.
In effect, within this limited preprinting formulation, black represents the practical absence of light. To attempt any further removal of light from such a color, in this kind of theoretical regime, is an oxymoron if not a conundrum.
In other words, in algorithms following this approach as noted previously it is without physical significance to propose application of a subtractive colorant—whether more black, or any other colorant—to such a full-scale application of black. In machine programming that adheres to this perceptual-color type of preprinting analysis, black ink alone is enough to consume all available degrees of freedom (even though a four-color-separation input signal has three more degrees of freedom not yet deployed).
Here not only has the verbatim specification of desired color been lost, as in the preceding section (e), but even before that step the preprinting process of translation into perceptual space has truncated the input amplitude. In other words the perceptual-theory system has entirely clipped the input signal to black, even though an important part of the input signal called for an overprinted tone in addition to the black.
Since a system so programmed must consider the desired color to be out-of-gamut, no amount of adjustment or tinkering can possibly represent that desired color within a conventional perceptual space. If the color rendition discussed in the preceding section was fairly described as “ruined”, then it remains to find adequate words for what is discussed here.
Since these special effects are difficult if not impossible to produce, it goes without saying that the printed hardcopy will not compare favorably with the results of a comparable project executed using a printing press. What is gone is that extra “punch” which the ink-thinker can now only visualize but never produce.
(g) Summary—In incremental printing, to prevent a grainy appearance black ink should not be used in midtone or highlight image regions. For a printer with an RGB-only interface, this desired preclusion of black ink is managed nicely through the colormap.
This route to blocking black ink out of highlight and midtone regions, however, is unworkable for a graphic-arts printer with a CMYK interface—or more generally for a printer that accepts four-color separations as inputs. The reason is that the artist or technician loses control over the amount of actual black ink generated.
In such a system, process black (CMY) inputs and actual black (K) inputs are caused to become indistinguishable. Naturally they come out the same, even though personnel preparing the ink specification have carefully designated them as different.
Any effort to colormap a preseparated CMYK specification necessarily always discards information about actual black ink. Depending on the specific colors called for, such an effort sometimes also clips the input color specification before even beginning the transformation.
In short, four-color specs may be fed, willy-nilly, into colormaps—but cannot meaningfully pass through colormaps. Conventional colormap techniques are not applicable to device-color CMYK incremental printers.
(h) Conclusion—As this discussion has shown, limitations in conventional incremental-printing color spaces continue to impede use of device-space specifications such as generated by craftspeople in the traditional printing industry—and thereby also continue to impede achievement of excellent hardcopy generation competitive with printing-press products. Thus important aspects of the technology used in the field of the invention are amenable to useful refinement.